For more than a decade, drilling tools as well as milling tools have been developed, e.g., in the form of shank-end mills, that, contrary to integral solid tools, are composed of two parts, viz. a basic body and a head being detachably connected with the same and thereby being replaceable, and in which head the requisite cutting edges are included. In such a way, the major part of the tool can be manufactured from a comparatively inexpensive material having a moderate modulus of elasticity, such as steel, while a smaller part, viz. the head, can be manufactured from a harder and more expensive material, such as cemented carbide, cermet, ceramics and the like, which gives the requisite cutting edges a good chip-removing capacity, a good machining precision and a long service life. In other words, the head forms a wear part that can be discarded after wear-out, while the basic body can be re-used several times (e.g., 10 to 20 replacements). A now recognized denomination of such cutting edge-carrying heads is “loose tops”, which henceforth will be used in this document.
On rotatable tools of the loose top type, a plurality of requirements are put, one of which is that torque should be transferable in a reliable way from the rotatable, driven basic body to the replaceable loose top. Furthermore, the basic body should, without problems, be able to carry the rearwardly directed, positive axial forces that the loose top may be subjected to during operation, e.g., drilling. Another requirement is that the loose top should be held centered in an exact and reliable way in relation to the basic body. Accordingly, each unintentional eccentricity between the center axis of the loose top and the center axis of the basic body should not be more than 0.01 mm. Most preferably, it should be smaller than 0.005 mm unless exact centricity can be achieved. A further requirement that is of particular importance as for drilling tools is that the loose top should be fixedly held to the basic body not only during drilling of a hole, but also during retraction of the drilling tool out of the same when the loose top is subjected to negative axial forces that aim to pull the same out of the jaw of the basic body. Yet another requirement or desire from the users' side is that the loose top should be mountable and dismountable in a rapid and convenient way without the basic body necessarily having to be removed from the driving machine. In addition, the tool, and in particular the loose top manufactured from an expensive material, should be possible to be manufactured at low costs.
Drilling tools as well as milling tools (shank-end mills) of the loose top type are widely known and may be divided into a number of different categories depending on the ideas on which the designs are based. Accordingly, there is a dividing line between a first category of tools that are based on the use of flexible, elastically bendable branches as drivers in the jaw of the basic body, and a second category of tools that utilize fixed, non-compliant lugs as drivers. In the first-mentioned category of tools, the branches are brought to be bent outward in connection with the mounting of the loose top, so as to then resiliently clamp the same in the jaw of the basic body. A general merit of such tools is that the clamping of the loose top in all essentials can be provided with the inherent elasticity of the branches. In other words, no separate tightening or lock features, e.g., in the form of screws are required.
A disadvantage of the fact that the drivers consist of flexible branches is, however, that the inherent elasticity of the branches runs the risk of being reduced after a certain time of use of the tool. When a number of loose tops have been replaced, the branches may accordingly start to be deformed plastically, rather than elastically, and in such a way successively lose their clamping force. There are many disadvantages with this. Above all, the loose top risks losing its exact centering in relation to the basic body. Furthermore, the ability of the drivers to counteract axial retraction of the loose top out of the jaw may be considerably impaired. This is particularly disadvantageous in connection with drilling tools, where the loose top risks remaining in the drilled hole in connection with the tool being pulled out of the same.
To the second category of tools, i.e., tools the torque-transferring drivers of which are non-compliant, belongs among others the drilling tool that is disclosed in U.S. Pat. No. 6,012,881. In this case, a rear coupling part of the loose top is inserted axially in a jaw between two non-compliant drivers, the insides of which include axially running, torque-transferring ridges that engage corresponding chutes in the coupling part of the loose top, besides which a centric pin protruding rearward from the loose top is inserted into a center hole that is formed in the basic body and mouths in the bottom of the jaw. With the pin, a radial screw mounted in a radial hole in the basic body co-operates with has the purpose of locking and centering the loose top in relation to the basic body.
A disadvantage of the drilling tool according to U.S. Pat. No. 6,012,881 is, however, that certain requirements on the tool are so conflicting that practical production of the same becomes difficult, not to say impossible. Accordingly, a fine fit is required between the concave and convex contact surfaces of the drivers and the coupling part, respectively, which have the purpose of centering the loose top, in order for the centering to become acceptable. The extremely fine fit between the contact surfaces means, however, that mounting as well as dismounting will be difficult to carry out. Another disadvantage is that all negative (retractive) axial forces that act on the loose top have to be carried by the radial screw alone via the pin. In order to manage this task, the radial screw has to be sturdy. What is more, also the fit between the centering pin of the loose top and the hole in the basic body has to be fine. Namely, if the fit is large (e.g., 0.1 mm), the radial screw will upon tightening apply radial forces to the pin and thereby the loose top that aim to space apart the loosetop from the centered position. If it is fine (e.g., about 0.01 mm), a frictional resistance difficult to overcome arises when the pin is to be pushed into and pulled out of, respectively, the hole.
The present invention aims at obviating the above-mentioned disadvantages of the known tool and at providing an improved loose-top tool, in particular a drilling tool. Therefore, an object of the invention to provide a loose-top tool in which the loose top in a reliable way can be retained in the jaw of the basic body even if the loose top is subjected to considerable negative axial forces. Furthermore, the invention aims at allowing an exceptionally exact centering of the loose top in relation to the basic body.
Another object of the invention is to provide a loose-top tool in which the loose top can be mounted in a simple and convenient way without taking into account any disadvantageous frictional resistance. Furthermore, the centering should be provided via the rear pin of the loose top, whereby the centering is made independent of other surfaces than co-operating contact surfaces on the pin and the receiving hole.